I was doing good finacialy with +100 gold per turn, but in one turn i was down to -50 gold per turn WTF?
how can it go down so fast?  
Im playing on KING, I was on a militay  campaign and destroyed evryone on my continent, i Razed most of the cities but puppeted ones with good resources.
Now my bulding maintence is over 200, on my own cities i only have very few buildings that i need, so all this maintence was comming from the puppeted cities?
So what do i do now, anex them all and then raze them?

Ive noticed that if you go into a negative gold output the puppeted states will build market and such the next time around. So if you got the cash I suppose you can wait it out.
 
Dunno what time you are at, but if you havnt picked the Order policy you should. It is awesome for big empires.
Other than that you can start building trading posts on farm tiles.
 
Either that or burn em.

Units have maintenance too, maybe it's that?

Roads too. Minimize the captured road network. Plus ecerything the others have said already.

My unit maintence is very low, only 40 gold,  I have Autocracy policy. The main killer is the building maintence. But now i waited a few turns and my gold is rising again, I did'nt do anything lol? sometimes i don't get this game.
 
EDIT - Ok so it was due to unhapainess.  I had 1 unhappiness, this caused my gold output from cities to  plummet.

I suffocate all my puppet cities by building trading posts all around them, thus they take longer time prodcing useless crap and actually make money since they have to work the tiles with trading posts on them. Ths also makes them gro slower so population unhapiness is less of a problem.
 
As for sucha  huge drop this cn only happen in 2 ways I believe:
 
1. Your trade agreements run out do if you were getting a lot of gold per turn this can put you into negative but this is not usually in such large amounts of 150g per turn
2. Your Golden Age has ended, so all your tiles that were making +1 gold, don't anymore, when you have a lot of cities and factor in a lot of % based bonuses this can easily add up to 150g per turn for large empires.

" My unit maintence is very low, only 40 gold,  I have Autocracy policy. The main killer is the building maintence. But now i waited a few turns and my gold is rising again, I did'nt do anything lol? sometimes i don't get this game.  EDIT - Ok so it was due to unhapainess.  I had 1 unhappiness, this caused my gold output from cities to  plummet. "

If I get into a major war and take over several cities, I try to leave them as puppet states for a few turns and then do one city at a time to keep my happiness up. I play a strong economy game, so the unhappiness is a killer to me. In my current game I got into a war and on the peace treaty negotiation the AI offered me EVERY one of his cities except for one. I took it, but all of a sudden I had a whole empire's worth of unhappy cities. Since I focus on economy a lot, I purchased some happiness focused buildings and then annexed the first city. After the courthouse was built there, I moved to the next. I'm about half way through the annexing process and it has kept my happiness and economy strong and my production is through the roof.